Beta Carotene is synthesized from isopentenyl diphosphate (produced in plastids from pyruvate and glyceraldehyde-3-phosphate). Isopentenyl diphosphate is converted to gernayl gernayl diphophate (GGPP). With the condensation of two GGPP, the cell is committed to produce beta-carotene (as well as the retinal carotenoids lutein and zeaxanthin). Phytoene synthase desaturates the GGPP to form the phytoene. Phytoene desaturase adds additional double bonds in the formation of lycopene (Ye, et.al). When lycopene is formed there is a branch in the pathway that leads either to the lutein or to the Beta-carotene and Zeaxanthin. The Beta-carotene is than cut or broken and initially the hydroxol group (which is than reformed) is reduced to produce Vitamin A. The conjugated double bonds of beta-carotene are the source of the color associated with the molecule. The greater number of double bonds allow the electrons to shift to a lower unoccupied molecular orbital thereby shifting the wavelength absorption from the ultra violet into the visible region. The progress of Vitamin A biosynthesis can be monitored spectrophotometrically (Barker,et.al).
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Figure 6: Diagram of the carotenoid pathway, note that the GPP is the committed step |
Figure 10: How beta-carotene is made in plants. To view some structures of intermediates in
the pathway: Isopentyl
Diphosphate |
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